There are continuing efforts in the coal industry to develop technologies resulting in fuels derived from coal which, as compared to raw coal, burn cleaner, have higher heat (BTU) content, and are more cost-efficient to transport. In coal industry parlance, such technologies are referred to as "clean coal" technologies.
Due to the plentiful reserves of low sulfur low rank coals, one area of development related to clean coal technologies is "thermally beneficiated low rank coal." This term means coal which has been processed at elevated temperatures to generate a product with a reduced moisture content and a higher heat value per unit of weight.
Such thermally beneficiated low rank coals have shown a tendency to spontaneously combust. Although raw coal also has a tendency to spontaneously combust, this tendency in raw coal is much less pronounced than that exhibited by thermally beneficiated low rank coals. This problem impedes the commercialization of thermally beneficiated low rank coals, because it does not allow them to be stored, shipped and handled using the same techniques used with raw coal.
The present invention addresses this problem and provides a method to stabilize commercial scale quantities of thermally beneficiated low rank coals against spontaneous combustion to a degree whereby they can be handled in a manner similar to raw coal. The term stability used herein is defined as the resistance to spontaneous combustion and the term stabilization is defined as processes which produce the resistance to spontaneous combustion.
It is to be understood that the term "coal," as used herein, shall include but not be limited to, peat, lignite, sub-bituminous and bituminous ranked coals. However, the beneficiated coal primarily contemplated by this invention is thermally beneficiated sub-bituminous and lignite coal.
Coal has a tendency to spontaneously heat and combust after it is mined. This tendency is exhibited when the coal is stored in large piles; in rail cars, storage silos, storage bunkers or in like storage facilities. Spontaneous heating and combustion of coal is the result of a combination of heat released during surface oxidation and heat released by hydration, i.e. the absorption of moisture. Both the oxygen and moisture are supplied by atmospheric air. If the coal is stored in a manner in which heat from oxidation and hydration is generated faster than it can be dissipated, the temperature of the stored coal increases until the combustion temperature of the coal is reached and combustion occurs. The natural insulating qualities of the stored coal facilitates the retention of heat and its attendant spontaneous combustion. The coal industry has adapted itself to handle and use raw coal within the general constraints of the coals natural tendency to spontaneously heat and combust. One of the methods for preventing spontaneous combustion is to move or use the coal before it is allowed to sit in large storage for more than a week. For raw coals, this short storage time does not allow the temperature to the point were spontaneous combustion occurs.
The spontaneous combustion problem is exacerbated in the case of thermally beneficiated low rank coals. Some of the thermally beneficiated low rank coals have had a substantial portion of their internal water content removed; without the heat dissipation capacity supplied by the water in the parent coal, these coals have a tendency to spontaneously combust that is greater than that of raw coal. Many of the thermally beneficiated low rank coals can spontaneously combust within one or two days of being placed in a large storage pile.
To remove this barrier to the commercialization of the new thermally beneficiated low rank coals, they must be stabilized to inhibit spontaneous combustion. Ideally, they should be stabilized to the point where they have the same stability as raw coal. This will permit the new thermally beneficiated low rank coals to be used with the same handling systems and with the same handling procedures as raw coal, and will thereby greatly increase the practical value of these thermally beneficiated fuels.
The inventors recognized and faced the issue of spontaneous combustion in connection with operating a demonstration facility built to produce a thermally beneficiated low rank coal, SynCoal.RTM.. U.S. Pat. No. 4,810,258, issued Mar. 7, 1989, to Greene, describes the SynCoal.RTM. product. U.S. Pat. No. 4,725,337, issued Feb. 16, 1988, also to Greene, describes the process for making SynCoal.RTM.. This technology is referred to as the Advanced Coal Conversion Process (ACCP).
The ACCP technology was first used to produce SynCoal.RTM. in bench tests, and in a pilot plant operated in 1986, prior to the issuance of U.S. Pat. Nos. 4,725,337 and 4,810,258, described above. To further develop the ACCP technology, a 300,000 ton per year demonstration facility was constructed in 1990-92 at Western Energy Company's Rosebud Coal Mine near Colstrip, Mont. The United States Department of Energy supported the ACCP Project through its Clean Coal Technology Program. One of the ultimate objective of the Clean Coal Program is to foster the commercialization of projects that provide fuels with characteristics that allow them to replace imported, higher cost fuels, thereby reducing dependence on imported fuels.
The problem of the spontaneous combustion tendency of SynCoal.RTM., was recognized during initial operations at the demonstration facility. Spontaneous combustion occurred within days of placing SynCoal.RTM. in air permeable storage silos or in open piles.
By repeating ACCP pilot tests in 1992, it was shown that the 1986 pilot plant produced SynCoal.RTM. which was equal in reactivity to that of the demonstration facility. The spontaneous heating characteristic was not identified at the pilot plant stage because the pilot plant generated SynCoal.RTM. in smaller quantities and at a lower rate than the demonstration facility. This low rate of production allowed enough time for the beneficiated coal to stabilize passively prior to it being covered by subsequent layers of SynCoal.RTM..
As an initial remedy to this problem of spontaneous combustion, a technique of "pile management", i.e. periodic handling and moving of the SynCoal.RTM. stored in piles or bins was developed. Based on actual observations, syncoalo spread at depths of less than 18 inches reached a peak temperature within approximately 2 days. High heat production was sustained for approximately 10 days, followed by a period of steady decline in pile temperatures. After being piled and held for over 3 months, spontaneous combustion did not occur, and apparently, a stable coal product was achieved. These results indicated that stability can be achieved through pile management, allowing oxidation and rehydration to occur along with sufficient heat dissipation.
By expanding on the concept of pile management, the inventors proceeded to develop a stabilization process from a bench scale to pilot scale. The inventors piloted a 1,000 pounds per hour process that produced air stabilized SynCoal.RTM. with about seven day stability. It remained a thermally beneficiated coal and retained its higher heat value per unit of weight.
The present invention stabilizes coal by using hot air or air with a reduced oxygen concentration to oxidize reactive sites on the surface of the coal. The oxidation step is followed by the addition of moisture to the coal product to bring the coal to a stable moisture level. Once the reactive sites of the coal have been oxidized and the coal adequately hydrated, the coal is stabilized and spontaneous combustion retarded. The adjustment of final product moisture content may be omitted if a lower moisture coal is desired and a less stable coal is acceptable.
The subject invention does not claim the novelty of oxidizing thermally beneficiated coals followed by rehydration. This invention teaches industrial scale methods of completing the stabilization including knowledge of maximum processing temperatures that may be utilized that minimizes the risk of process fires and the duration of processing necessary to obtain a stability level that allows handling and transporting the product using conventional means.
Fortunately, 100% stability is not required, only stability that will allow handling in a manner similar to raw coals, which allows for up to 7 days before use or rehandling. In general this 7 days before use is the time-frame meant to be comparable to raw coal used in commercial application.
Economical commercial application of oxidative stabilization requires the smallest possible reaction chamber in order to minimize construction and operating costs. If the processing can be completed in less time, the processing equipment can be scaled down resulting in reduced equipment costs and reduced operating costs.